1. Field of the Invention
The present invention relates to a method of measuring warpage of a rear surface of a substrate used in a semiconductor device or the like, and more specifically, to a method of measuring warpage of a rear surface (a surface opposite to a crystal growth surface; hereinafter the same applies) of a substrate using a laser displacement meter.
2. Description of the Background Art
In a substrate used in a semiconductor device or the like, it is necessary to form one or more semiconductor layers having good quality on the substrate in order to obtain a semiconductor device having excellent properties. Accordingly, the substrate is required to have a crystal growth surface with reduced warpage and surface roughness. The warpage of the crystal growth surface can be measured by a flatness tester employing optical interferometry, and the surface roughness of the crystal growth surface can be measured by a 3D-SEM (three-dimensional scanning electron microscope; hereinafter the same applies) or the like (see for example “Superprecision Wafer Surface Control Technology” by Yoshiaki Matushita et al., the first edition, Science Forum Inc., Feb. 28, 2000, pages 258-264 and 272-278 (Non-Patent Document 1)).
In order to form one or more semiconductor layers having good quality on the substrate, the substrate is required to not only have a crystal growth surface with reduced warpage and surface roughness, but also have a rear surface with reduced warpage and surface roughness. If the rear surface has large warpage and surface roughness, this causes an increase in a gap portion formed between the rear surface of the substrate and a susceptor (meaning a table on which a substrate is disposed; hereinafter the same applies) when a semiconductor layer is formed on the crystal growth surface of the substrate. As a result, heat transferred from the susceptor to the substrate is unevenly distributed, and the semiconductor layer cannot be formed evenly and stably on the crystal growth surface of the substrate. Consequently, a semiconductor layer having good quality cannot be formed.
Consequently, in order to prepare a substrate suitable for fabrication of a semiconductor device, it is necessary to evaluate not only warpage and surface roughness of a crystal growth surface of a substrate but also warpage and surface roughness of a rear surface of the substrate. The surface roughness of the rear surface can be measured by a 3D-SEM or the like.
However, the rear surface has a surface roughness greater than that of the crystal growth surface, and it often has a surface roughness Ra of not less than 50 nm. Accordingly, it is difficult to measure the warpage of the rear surface by a flatness tester employing optical interferometry. Further, since the flatness tester employing optical interferometry cannot obtain a reflected beam, it cannot provide data analysis.
Therefore, there has been a strong need to develop a method of measuring warpage of a rear surface of a substrate in order to fabricate a semiconductor device having excellent properties.